This invention relates to a display arrangement using Focal Plane Arrays (FPAs), and in particular but not exclusively to an imager for use in the Infra Red (IR) wavebands using pyroelectric FPAs.
IR imaging systems are becoming more important in many fields now, in particular military, security and search and rescue applications. Early IR imagers employed a small number of detector elements, across which was scanned an IR image of the scene via a system of mirrors and polygons. More recent developments include imagers based on 2 dimensional arrays of detector elements so called staring arrays, which require no scanning to produce a useful image of the scene. The dwell time available for each detector element in such systems is considerably increased over earlier scanner systems resulting in significantly improved system performance being achievable from comparable detector materials. The IR system designer can choose whether to exploit this increase in performance or use a lower performance detector material to achieve a similar sensitivity as in the earlier scanner systems. High system performance is typified by imagers based on arrays of Cadmium Mercury Telluride cooled to liquid nitrogen temperatures, whilst conventional levels of performance are achieved by imagers based on pyroelectric and other bolometric detector arrays. These latter systems offer significant advantages in terms of cost and/or logistical support requirements (such as coolant supplies) over the high performance systems.
Applications of the more modest performance systems include personal battery powered imagers and rifle sights, and in general such systems utilise a cathode ray tube (CRT) for the display of information to the user. Such displays, although commonly used, suffer from considerable disadvantages. In particular, the CRTs are manufactured using vacuum glass technology, and as such are particularly fragile unless steps are taken to ruggedise the tube. Also likely to suffer damage from vibration and shock are the delicate electrodes and phosphor screen coatings. Although ruggedised CRTs are available they are expensive and more bulky than their conventional equivalents. In addition, the power consumption of CRTs is usually in excess of one watt, and can run to several watts for the larger and/or ruggedised tubes, not including the requirements of the drive electronics. A further problem associated with CRT displays is that the electronic drive circuits rarely operate with the same scan sequences as the detector, resulting in the requirement for frame storage and data resequencing--if only at different timings--between the detection and display processes.
Some early thermal imaging systems based on scanning mirrors, which sweep the image past a relatively small number of detectors, utilise the back of the mirror to sweep an image of a small array of LEDs across the field of view of the eye. Persistence of vision causes the image to appear continuous. For systems based on staring focal plane arrays, however, such mirror arrangements are not possible, since the array simply stares into space without the requirement for scanning mirrors etc. to detect an image.